Federal Guidelines for Dam Safety.pdf

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Selecting and Accomodating

Inow Design Floods for Dams

April 2004

Federal

GuidelinesforDamSafety


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FEDERAL GUIDELINES FOR DAM SAFETY:SELECTING AND ACCOMMODATINGINFLOW DESIGN FLOODS FOR DAMS

prepared by theINTERAGENCY COMMITTEE ON DAM SAFETY

U.S. DEPARTMENT OF HOMELAND SECURITYFEDERAL EMERGENCY MANAGEMENT AGENCYOCTOBER 1998

Reprinted April 2004


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PREFACE

In April 1977, President Carter issued a memorandum directing the review of federal dam safety activities by an

ad hoc panel of recognized experts. In June 1979, the ad hoc interagency committee on dam safety (ICODS)

issued its report, which contained the first guidelines for federal agency dam owners. The Federal Guidelines for

Dam Safety (Guidelines) encourage strict safety standards in the practices and procedures employed by federal

agencies or required of dam owners regulated by the federal agencies. The Guidelines address management practices and procedures but do not attempt to establish technical standards. They provide the most complete and

authoritative statement available of the desired management practices for promoting dam safety and the welfare of

the public.

To supplement the Guidelines, ICODS prepared and approved federal guidelines in the areas of emergency action

planning; earthquake analysis and design of dams; and selecting and accommodating inflow design floods for

dams. These publications, based on the most current knowledge and experience available, provided authoritativestatements on the state of the art for three important technical areas involving dam safety. In 1994, the ICODS

Subcommittee to Review/Update the Federal Guidelines began an update to these guidelines to meet new dam

safety challenges and to ensure consistency across agencies and users. In addition, the ICODS Subcommittee on

Federal/Non-Federal Dam Safety Coordination developed a new guideline, Hazard Potential Classification

System for Dams.

With the passage of the National Dam Safety Program Act of 1996, Public Law 104-303, ICODS and its

Subcommittees were reorganized to reflect the objectives and requirements of Public Law 104-303. In 1998, the

newly convened Guidelines Development Subcommittee completed work on the update of all of the following

guidelines:

•

Federal Guidelines for Dam Safety: Emergency Action Planning for Dam Owners

• Federal Guidelines for Dam Safety: Hazard Potential Classification System for Dams

•

Federal Guidelines for Dam Safety: Earthquake Analyses and Design of Dams

• Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams

• Federal Guidelines for Dam Safety: Glossary of Terms

The publication of these guidelines marks the final step in the review and update process. In recognition of the

continuing need to enhance dam safety through coordination and information exchange among federal and state

agencies, the Guidelines Development Subcommittee will be responsible for maintaining these documents and

establishing additional guidelines that will help achieve the objectives of the National Dam Safety Program.

The members of all of the Task Groups responsible for the update of the guidelines are to be commended for their

diligent and highly professional efforts:

Harold W. Andress, Jr.

Chairman, Interagency Committee on Dam Safety


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SELECTING AND ACCOMMODATING INFLOW DESIGN

FLOODS FOR DAMS

TASK GROUP

David Achterberg**

DOI, Bureau of Reclamation

Jerrold W. Gotzmer*

Federal Energy Regulatory Commission

Ronald Spath*

Federal Energy Regulatory Commission

Ming Tseng*U.S. Army Corps of Engineers

Donald E. Woodward*

USDA, Natural Resources Conservation Service

Norman Miller

USDA, Natural Resources Conservation Service

Sam A. Shipman

Tennessee Valley Authority

*Chair of Task Group that completed final document

**Member of Task Group that completed final document


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TABLE OF CONTENTS

I. INTRODUCTION ..............................................................................................................................1

A. Purpose ...............................................................................................................................1

B. Background .............................................................................................................................1

1. Early Periods.................................................................................................................12. Transition .....................................................................................................................23. Current Practices ..........................................................................................................2

C. Scope .......................................................................................................................................3

1. General .........................................................................................................................32. Philosophy and Principles ............................................................................................3

3. Content .........................................................................................................................4

II. DEFINITIONS OF TERMS ............................................................................................................5

III. SELECTING INFLOW DESIGN FLOODS ..............................................................................11

A. Introduction............................................................................................................................11B. Hazard Potential Evaluation ..................................................................................................11

1. General ........................................................................................................................12

2. Hydrologic Modes of Failure .....................................................................................12a. Overtopping.....................................................................................................12

b. Erosion in Earth Spillways..............................................................................12

c. Overstressing of Structural Components .......................................................133. Defining the Hazard Potential ....................................................................................13

4. Evaluating the Consequences of Dam Failure ...........................................................14

5. Studies to Define the Consequences of Dam Failure .................................................156. Incremental Hazard Evaluation for Inflow Design Flood Determination .................17

7. Criteria for Selecting the Inflow Design Flood...........................................................19

C. PMFs for Dam Safety ............................................................................................................21

1. General ........................................................................................................................212. PMP.............................................................................................................................21

D. Floods To Protect Against Loss of Benefits During the Life of the Project-

Applicable Only to Low-Hazard Dams .............................................................................22

IV. ACCOMMODATING INFLOW DESIGN FLOODS ...............................................................23

A. Introduction ...........................................................................................................................231. General .......................................................................................................................23

2. Guidelines for Initial Elevations ................................................................................23

3. Reservoir Constraints .................................................................................................23

4. Reservoir Regulation Requirements ..........................................................................235. Evaluation of Domino-like Failure ............................................................................24

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TABLE OF CONTENTS (Continued)

B. Spillway and Flood Outlet Selection and Design ..................................................................25

1. General .......................................................................................................................252. Gated or Ungated Spillways ......................................................................................25

3. Design Considerations ...............................................................................................26

C. Freeboard Allowances ...........................................................................................................271. General .......................................................................................................................27

2. Freeboard Guidelines .................................................................................................28

Appendix A - References by Top ....................................................................................................... A-1

Appendix B – Inflow Design Flood Selection Procedures

Figure 1. Illustration of Incremental Increase Due to Dam Failure

Figure 2. Illustration of Incremental Area Flooded by Dam FailureFigure 3. Regions Covered by PMP Studies

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I. INTRODUCTION

A. Purpose

The purpose of these Guidelines is to provide thorough and consistent procedures forselecting and accommodating Inflow Design Floods (IDFs). The IDF is the flood flow

above which the incremental increase in water surface elevation downstream due to

failure of a dam or other water retaining structure is no longer considered to present an

unacceptable additional downstream threat.

B. Background

Current practice in the design of dams is to use the IDF that is deemed appropriate for thehazard potential of the dam and reservoir, and to design spillways and outlet works that

are capable of safely accommodating the floodflow without risking the loss of the dam or

endangering areas downstream from the dam to flows greater than the inflow. However,

there are many dams whose discharge capabilities were designed using methods that arenow considered unconservative and potentially unsafe.

Inflow design flood selection began primarily as a practical concern for protection of a

dam and the benefits it provides. The early 1900's saw an increase in the Nation's social

awareness that was demonstrated by various legislative acts designed to protect the publicfrom certain high risk activities. The same era witnessed an increase in the number and

size of dams built. When the "big dam" era began in the 1930's, safety clearly became a

more dominant factor. It was recognized that dams needed to be designed to

accommodate water flows that might be greater than the anticipated "normal" flow.

1. Early Periods. Before 1900, designers of dams had relatively little hydrologic data ortools to indicate flood potential at a proposed dam site. Estimates of flood potential were

selected by empirical techniques and engineering judgment based on high water marks orfloods of record on streams being studied.

Later, engineers began examining all past flood peaks in a region to obtain what washoped to be a more reliable estimate of maximum flood-producing potentials than alimited record on a single stream. Designers would base their spillway design on these

estimates, sometimes providing additional capacity as a safety factor. Some spillways

were designed for a multiple of the maximum known flood, for example, twice the

maximum known flood. The multiples and safety factors were based on engineering

judgment; the degree of conservatism in the design was unknown.

By the 1930's, it became apparent that this approach was inadequate. As longer

hydrologic records were obtained, new floods exceeded previously recorded maximumfloods. With the introduction of the unit hydrograph concept by Sherman in 1933, it

became possible to estimate floodflows from storm rainfall. The design of dams began to

be based upon the transposition of major storms that had occurred within a region, i.e.,

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transfer and centering of relevant storm rainfall patterns over the basin above the dam site being evaluated. It was recognized that flood peaks are dependent on topography, size of

individual watersheds, and chance placement of the storm's center over the watershed. In

addition, within meteorologically similar areas, observed maximum rainfall values could

provide a better indication of maximum flood potentials than data on flood dischargesfrom individual watersheds. If, in the judgment of the designer, the storm was not

representative of what might occur, rainfall amounts were increased to represent a more

severe event, and the dam was designed accordingly.

2. Transition. Engineers next turned to hydrometeorologists to determine if upper limitsfor rates of precipitation could be established on a rational basis. Careful consideration

was given to the meteorology of storms that produced major floods in various parts of thecountry. The large scale features of the storm and measures of atmospheric moisture,

such as dewpoint temperatures, were considered as well as the rainfall depth-area-

duration values produced by these storms. It was then possible to increase the storm

dewpoint temperature and other factors affecting rainfall to the maximum appropriatevalues. This increase resulted in estimates of probable maximum precipitation (PMP),

and thus introduced the concept of a physical upper limit to precipitation. When

translated to runoff, the estimated floodflow is known as the probable maximum flood(PMF).

At first, the terms maximum possible precipitation and maximum possible flood were

used. However, the terminology was changed to probable maximum to recognize theuncertainties in the estimates of the amount of precipitation, and the most severe

combination of critical meteorological and hydrologic conditions that are reasonably

possible in the region. Today, the PMF is generally accepted as the standard for the safety

design of dams where the incremental consequences of failure have been determined to be unacceptable.

In the late 1940's, the ability to estimate the consequences of dam failure, including the

loss of life, was still quite limited. The height of the downstream flood wave and theextent of wave propagation were known to be a function of dam height and reservoir

volume. Thus, early standards for dam design were based upon the size of the dam in

terms of its height, the reservoir storage volume, and the downstream development.

The practice of setting inflow design flood standards based upon the size of a dam, its

reservoir volume, and current downstream development resulted in an inconsistent level

of design throughout the country. The determination of the consequences of a dam failureis more complex than can be evaluated by these simple relationships.

3. Current Practices. In 1985, the National Academy of Sciences (NAS) published astudy of flood and earthquake criteria which contained an inventory of current practices

in providing dam safety during extreme floods. The inventory showed considerable

diversity in approach by various federal, state, and local government agencies,

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professional societies, and private firms. While the inventory shows a fair consensus onspillway requirements for dams having a high-hazard potential, there is a wide range of

criteria being applied to dams with lower hazard classifications.

Several observations about the evaluation of hydrologic conditions were made in the NAS study. Use of PMP for evaluating spillway capacity requirements for large, high-

hazard dams predominates, although some state agencies have standards that do notrequire such dams to pass the full estimated PMF based on the PMP. The influence of the

principal federal dam-building and dam safety agencies is evident in the majority of the

standards for large, high-hazard dams, but the practices of those agencies have had lesseffect on current state standards for small dams in less hazardous situations.

As a result of inspections authorized by Public Law 92-367, the National Dam InspectionAct, and carried out by the U.S. Army Corps of Engineers from 1977 to 1981, several

states have adopted the spillway capacity criteria used in those inspections. Several other

states have adopted the standards used by the Natural Resources Conservation Service(formerly the Soil Conservation Service) for the design of smaller dams constructed

under that agency's programs.

Most agencies draw a distinction between design criteria that are applied to existing dams

and those that are applied to new dams. However, because dam failures present the same

consequences to life and property, it is desirable that existing dams meet the criteriaestablished for new dams.

Today, hydrologically safe designs should be based on current state-of-the-art criteria.

Now that engineers can estimate downstream flood levels resulting from dam failure,

safety design standards can be tied specifically to a detailed evaluation of the impacts of a

flood if a dam were to fail. Although debate continues over the proper criteria and degreeof conservatism warranted when evaluating and designing modifications to existing

dams, and when designing new dams, criteria used by dam designers, regulators, andowners now focus on ensuring public safety.

C. Scope

1. General. These Guidelines are not intended to provide a complete manual of all

procedures used for estimating inflow design floods; the selection of procedures isdependent upon available hydrologic data and individual watershed characteristics. All

studies should be performed by an engineer experienced in hydrology and hydraulics,

directed and reviewed by engineers experienced in dam safety, and should contain a

summary of the design.

2. Philosophy and Principles. The basic philosophy and principles are described insufficient detail to achieve a reasonable degree of uniformity in application, and to

achieve a consistent and uniform nationwide treatment among federal agencies in thedesign of dams from the standpoint of hydrologic safety.

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3. Content. The following topics are discussed in these Guidelines:

• Selecting the IDF - The selection of the appropriate IDF for a dam is related to the

hazard potential classification and is the result of the incremental hazard

evaluation.

• Accommodating the IDF - Site-specific considerations are necessary to establishhydrologic flood routing criteria for each dam and reservoir. The criteria for

routing the IDF or any other flood should be consistent with the reservoir

regulation procedure that is to be followed in actual operation.

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Flood Plain: The downstream area that would be inundated or otherwise affected by the

failure of a dam or by large flood flows.

Flood Routing: A process of determining progressively the amplitude of a flood wave as

it moves past a dam and continues downstream.

Flood Storage: The retention of water or delay of runoff either by planned operation, as

in a reservoir, or by temporary filling of overflow areas, as in the progression of a flood

wave through a natural stream channel.

Freeboard: Vertical distance between a specified stillwater reservoir surface elevation

and the top of the dam, without camber.

Foundation: The portion of the valley floor that underlies and supports the dam

structure.

Gate: A movable water barrier for the control of water.

Hazard: A situation which creates the potential for adverse consequences such as loss of

life, property damage, or other adverse impacts. Impacts in the area downstream of a damare defined by the flood waters released through spillways and outlet works of the dam or

waters released by partial or complete failure of the dam. There may also be impacts

upstream of the dam due to backwater flooding or landslides around the reservoir

perimeter.

Hazard Potential Classification: A system that categorizes dams according to the

degree of adverse incremental consequences of a failure or misoperation of a dam. TheHazard Potential Classification does not reflect in any way on the current condition of the

dam (i.e., safety, structural integrity, flood routing capacity).

Hydrograph, Flood: A graphical representation of the flood discharge with respect to

time for a particular point on a stream or river.

Hydrology: One of the earth sciences that encompasses the natural occurrence,

distribution, movement, and properties of the waters of the earth and their environmentalrelationships.

Hydrometeorology: The study of the atmospheric and land-surface phases of thehydrologic cycle with emphasis on the interrelationships involved.

Inflow Design Flood (IDF): The flood flow above which the incremental increase in

downstream water surface elevation due to failure of a dam or other water impounding

structure is no longer considered to present an unacceptable additional downstream

threat. The IDF of a dam or other water impounding structures is the flood hydrograph

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used in the design or evaluation of a dam, its appurtenant works, particularly for sizingthe spillway and outlet works, for determining maximum height of a dam, freeboard, and

flood storage requirements. The upper limit of the IDF is the probable maximum flood.

Intake: Placed at the beginning of an outlet-works waterway (power conduit, watersupply conduit), the intake establishes the ultimate drawdown level of the reservoir by the position and size of its opening(s) to the outlet works. The intake may be vertical or

inclined towers, drop inlets, or submerged, box-shaped structures. Intake elevations are

determined by the head needed for discharge capacity, storage reservation to allow forsiltation, the required amount and rate of withdrawal, and the desired extreme drawdown

level.

Inundate: To overflow, to flood.

Inundation Map: A map showing areas that would be affected by flooding from an

uncontrolled release of a dam's reservoir.

Landslide: The unplanned descent (movement) of a mass of earth or rock down a slope.

Maximum Wind: The most severe wind for generating waves that is reasonably possible

at a particular reservoir. The determination will generally include results of meteorologicstudies which combine wind velocity, duration, direction, fetch, and seasonal distribution

characteristics in a realistic manner.

Meteorological Homogeneity: Climates and orographic influences that are alike or

similar.

Meteorology: The science that deals with the atmosphere and atmospheric phenomena,

the study of weather, particularly storms and the rainfall they produce.

One-Percent-Chance Flood: A flood that has 1 chance in 100 of being equaled or

exceeded during any year.

Orographic: Physical geography that pertains to mountains and to features directly

connected with mountains and their general effect on storm path and generation ofrainfall.

Outlet Works: A dam appurtenance that provides release of water (generally controlled)from a reservoir.

Probable Maximum Flood (PMF): The flood that may be expected from the most

severe combination of critical meteorologic and hydrologic conditions that are reasonably

possible in the drainage basin under study.

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Probable Maximum Precipitation (PMP): Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area

at a particular geographical location during a certain time of the year. Reservoir

Regulation Procedure (Rule Curve): Refers to a compilation of operating criteria,

guidelines, and specifications that govern the storage and release function of a reservoir.It may also be referred to as operating rules, flood control diagram, or water control

schedule. These are usually expressed in the form of graphs and tabulations,

supplemented by concise specifications and are often incorporated in computer programs.In general, they indicate limiting rates of reservoir releases required or allowed during

various seasons of the year to meet all functional objectives of the project.

Reservoir Rim: The boundary of the reservoir including all areas along the valley sides

above and below the water surface elevation associated with the routing of the IDF.

Risk: The relationship between the consequences resulting from an adverse event and its

probability of occurrence.

Risk-based Analysis: A procedure in which the consequences and risks of adverse

events and alternatives for mitigation are evaluated and arranged in a manner thatfacilitates a decision on the action to be taken. A risk-based analysis may be the basis for

the selection of the IDF for a particular dam.

Seiche: An oscillating wave in a reservoir caused by a landslide into the reservoir or

earthquake-induced ground accelerations or fault offset.

Sensitivity Analysis: An analysis in which the relative importance of one or more of the

variables thought to have an influence on the phenomenon under consideration isdetermined.

Settlement: The vertical downward movement of a structure or its foundation.

Significant Wave Height: Average height of the one-third highest individual waves.

May be estimated from wind speed, fetch length, and wind duration.

Spillway: A structure over or through which flow is discharged from a reservoir. If the

rate of flow can be controlled by mechanical means, such as gates, it is considered acontrolled spillway. If the geometry of the spillway is the only control, it is considered an

uncontrolled spillway. Definitions of specific types of spillways follow:

Service Spillway: A spillway that is designed to provide continuous or frequentregulated or unregulated releases from a reservoir without significant damage to

either the dam or its appurtenant structures. This is also referred to as principal

spillway.

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Auxiliary Spillway: Any secondary spillway which is designed to be operatedinfrequently, possibly in anticipation of some degree of structural damage or

erosion to the spillway that would occur during operation.

Emergency Spillway: A spillway that is designed to provide additional protection against overtopping of dams, and is intended for use under extremeflood conditions or misoperation or malfunction of the service spillway and/or the

auxiliary spillway.

Spillway Capacity: The maximum spillway outflow which a dam can safely pass with

the reservoir at its maximum level.

Stillwater Level: The elevation that a water surface would assume if all wave actions

were absent.

Storm: The depth, area, and duration distributions of precipitation.

Storm Center: Location of the storm pattern such that the precipitation falls on a specific

drainage basin to create the runoff at the site under consideration.

Storm Transposition: The application of a storm from its actual location of occurrence

to some other area within the same region of meteorological homogeneity. Storm

transposition requires the determination of whether the particular storm could occur over

the area to which it is to be transposed.

Surcharge: The volume or space in a reservoir between the controlled retention water

level and the maximum water level. Flood surcharge cannot be retained in the reservoir but will flow out of the reservoir until the controlled retention water level is reached.

Toe of the Dam: The junction of the downstream slope or face of a dam with the ground

surface; also referred to as the downstream toe. The junction of the upstream slope with

the ground surface is called the heel or the upstream toe.

Topographic Map: A detailed graphic delineation (representation) of natural and man-

made features of a region with particular emphasis on relative position and elevation.

Tributary: A stream that flows into a larger stream or body of water.

Unit Hydrograph: A hydrograph with a volume of one inch of direct runoff resulting

from a storm of a specified duration and areal distribution. Hydrographs from otherstorms of the same duration and distribution are assumed to have the same time base but

with ordinates of flow in proportion to the runoff volumes.

Watershed: The area drained by a river or river system.

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Wave Runup: Vertical height above the stillwater level to which water from a specific

wave will run up the face of a structure or embankment.

Wind Setup: The vertical rise of the stillwater level at the face of a structure or

embankment caused by wind stresses on the surface of the water.

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III. SELECTING INFLOW DESIGN FLOODS

A. Introduction

Many thousands of dams have been constructed in the United States, and new dams continue to

add to this total. The proper operation of dams to withstand natural forces, including extreme

hydrologic events, is an important matter of public safety and concern.

In today's technical world, extreme hydrologic events resulting in dam failures are classified as"low-probability, high-consequence" events. In addition, the potential for losses due to increased

downstream development may increase the consequences of a dam failure.

There has been a growing concern and increased attention to dam safety over the past two

decades, primarily as a result of a number of catastrophic dam failures. As a result, the inspection

of non-federal dams authorized by Public Law 92-367, the National Dam Inspection Act,identified some 2,900 unsafe dams of which 2,350 had inadequate spillway capacities. Since

there are approximately 23,772 high and significant-hazard dams in the present NationalInventory of Dams, the number of dams which have inadequate spillways could be significantlyhigher.

The adequacy of a spillway must be evaluated by considering the hazard potential, which wouldresult from failure of the project works during flood flows. (See Chapter II for a definition of

hazard potential.) If failure of the project works would present an unacceptable downstream

threat, the project works must be designed to either withstand overtopping for the loadingcondition that would occur during a flood up to the probable maximum flood, or to the point

where a failure would no longer cause an unacceptable additional downstream threat.

The procedures used to determine whether or not the failure of a project would cause anunacceptable downstream threat vary with the physical characteristics and location of the project,

including the degree and extent of development downstream.

Analyses of dam failures are complex, with many historical dam failures not completelyunderstood. The principal uncertainties in determining the outflow from a dam failure involve

the mode and degree of failure. These uncertainties can be circumvented in situations where it

can be shown that the complete and sudden removal of the dam would not result in unacceptableconsequences. Otherwise, reasonable failure postulations and sensitivity analyses should be used.

Suggested references regarding dam failure studies are listed in Appendix A. If it is judged that a

more extensive mode of failure than that normally recommended for the type of structure under

investigation is possible, then analyses should be done to determine whether remedial action isrequired. Sensitivity studies on the specific mode of failure should be performed when failure is

due to overtopping.

B. Hazard Potential Evaluation

A properly designed, constructed, and operated dam can be expected to improve the safety ofdownstream developments during floods. However, the impoundment of water by a dam can

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create a potential hazard to downstream developments greater than that which would existwithout the dam because of the potential for dam failure. There are several potential causes of

dam failure, including hydrologic, geologic, seismic, and structural.

These Guidelines are limited to the selection of the inflow design flood (IDF) for the hydrologic

design of a dam.

1. General. Once a dam is constructed, the downstream hydrologic regime may change, particularly during floods. The change in hydrologic regime could alter land use patterns to

encroach on a flood plain that would otherwise not be developed without the dam.

Consequently, evaluation of the consequences of dam failure must be based on the dam

being in place, and must compare the impacts of with-failure and without-failure

conditions on existing development and known and prospective future development when

evaluating the downstream hazard potential.

2. Hydrologic Modes of Failure. Many dam failures have resulted because of an inability tosafely pass flood flows. Failures caused by hydrologic conditions can range from sudden, withcomplete breaching or collapse, to gradual, with progressive erosion and partial breaching.

The most common modes of failure associated with hydrologic conditions include overtop-ping,erosion of earth spillways, and overstressing the dam or its structural components. The following

paragraphs describe briefly each of the modes of failure caused by hydrologic conditions.

a. Overtopping. Overtopping of a dam occurs when the water level in the reservoir exceeds the

height of the dam and flows over the crest. Overtopping will not necessarily result in a failure.Failure depends on the type, composition, and condition of the dam and the depth and duration of

flow over the dam.

Embankment dams are very susceptible to failure when overtopped because of potential erosion.

If the erosion is severe, it can lead to a breach and failure of the dam. During overtopping, the

foundation and abutments of concrete dams also can be eroded, leading to a loss of support andfailure from sliding or overturning. In addition, when a concrete dam is subjected to overtopping,

the loads can be substantially higher than those for which the dam was designed. If the increased

loading on the dam itself due to overtopping is too great, a concrete dam can fail by overturningor sliding.

b. Erosion in Earth Spillways. High or large flows through earthen spillways adjacent to damscan result in erosion that progresses to the dam and threatens it. Erosion can also cause

headcutting that progresses toward the spillway crest and eventually leads to a breach.

Discontinuities in slope, nonuniform vegetation or bed materials, and concentrated flow areascan start headcuts and accelerate the erosion process. Flood depths that exceed the safe design

parameters can produce erosive forces that may cause serious erosion in the spillway.

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Erosion that occurs due to flow concentrations can start where roads or trails are devoid ofvegetation or have ruts that run parallel to the spillway flow. A varied mix of earth materials,

unlevel cross sections, uneven stands of vegetation, and obstructions such as trash accumulations

can cause turbulent, concentrated flow conditions that start gullies that can widen and migrate

upstream to breach the spillway crest.

Runoff brought into a spillway channel by a side inlet may also disrupt the desirable uniformflow pattern and increase the erosion in the channel.

The probability of failure of an earthen auxiliary spillway due to erosion is increased when thecapacity of the service spillway inlets (outlet works) or gates are reduced due to trash

accumulations. These accumulations reduce the available capacity through these appurtenances

and increase the volume, depth, frequency, and duration of flow in the auxiliary spillway.

c. Overstressing of Structural Components. As flood flows enter the reservoir, the reservoir

will normally rise to a higher elevation. Even though a dam (both concrete and earthembankment dams) may not be overtopped, the reservoir surcharge will result in a higherloading condition. If the dam is not properly designed for this flood surcharge condition, either

the entire dam or the structural components may become overstressed, resulting in an

overturning failure, a sliding failure, or a failure of specific structural components (such as theupstream face of a slab and buttress dam). Embankment dams may be at risk if increased water

surfaces result in increased pore pressures and seepage rates, which exceed the seepage control

measures for the dam.

3. Defining the Hazard Potential. The hazard potential is the possible adverse incremental

consequences that result from the release of water or stored contents due to failure of the dam or

misoperation of the dam or appurtenances. Hazard potential does not indicate the structuralintegrity of the dam itself, but rather the effects if a failure should occur. The hazard potential

assigned to a dam is based on consideration of the effects of a failure during both normal andflood flow conditions.

Dams assigned the low hazard potential classification are those where failure or misoperationresults in no probable loss of human life and low economic and/or environ-mental losses. Losses

are principally limited to the owner's property.

Dams assigned the significant hazard potential classification are those dams where failure or

misoperation results in no probable loss of human life but can cause economic loss,

environmental damage, disruption of lifeline facilities, or can impact other concerns. Significanthazard potential classification dams are often located in predominantly rural or agricultural areas

but could be located in areas with population and significant infrastructure.

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Dams assigned the high hazard potential classification are those where failure or misoperationwill probably cause loss of human life.

The hazard potential classification assigned to a dam should be based on the worst-case

failure condition, i.e., the classification is based on failure consequences resulting from the

failure condition that will result in the greatest potential for loss of life and property

damage. For example, a failure during normal operating conditions may result in the releasedwater being confined to the river channel, indicating a low-hazard potential. However, if the dam

were to fail during a floodflow condition, and the resultant incremental flood flow would be a

potential loss of life or serious damage to property, the dam would have high-hazard potentialclassification.

In many cases, the hazard potential classification can be determined by field investigations and areview of available data, including topographic maps. However, when the hazard potential

classification is not apparent from field reconnaissance, detailed studies, including dambreak

analyses, are required. These detailed studies are required to identify the floodflow conditionabove which the additional incremental increase in elevation, due to failure of a dam, is no

longer considered to present an unacceptable threat to downstream life and property.

The hazard potential classification of a project determines the level of engineering review and

the criteria that are applicable. Therefore, it is critical to determine the appropriate hazard

potential of a dam because it sets the stage for the analyses that must be completed to properlyevaluate the integrity of any dam.

4. Evaluating the Consequences of Dam Failure.

There have been about 200 notable dam failures resulting in more than 8,000 deaths in the

Twentieth Century. Dam failure is not a problem confined to developing countries or to acompilation of past mistakes that are unlikely to occur again.

Many dam owners have a difficult time believing that their dams could experience a rainfallmany times greater than any they have witnessed over their lifetimes. Unfortunately, this attitude

leads to a false sense of security because floods much greater than those experienced during anyone person's lifetime can and do occur.

Estimates of the potential for loss of human life and the economic impacts of damage resulting

from dam failure are the usual bases for defining hazard potential. Social and environmental

impacts, damage to national security installations, and political and legal ramifications are noteasily evaluated, and are more susceptible to subjective or qualitative evaluation. Because their

actual impacts cannot be clearly defined, particularly in economic terms, their consideration as

factors for determining the hazard potential must be on a case-by-case basis.

In most situations, the investigation of the impacts of failure on downstream life and property is

sufficient to determine the appropriate hazard potential rating and to select the appropriate IDFfor a project. However, in determining the appropriate IDF for a project, there could be

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circumstances beyond loss of life and property damage, particularly when a failure would haveminimal or no impact on downstream life and property, which would dictate using a more

conservative hazard potential and IDF. For example, the reservoir of a dam that would normally

be considered to have a low-hazard potential classification based on insignificant incremental

increases (in elevation) due to a failure may be known to contain extensive toxic sediments. Ifreleased, those toxic sediments would be detrimental to the ecosystem. Therefore, a low-hazard

potential classification would not be appropriate. Instead, a higher standard should be used for

classifying the hazard potential and selecting the IDF.

5. Studies To Define the Consequences of Dam Failure.

The degree of study required to sufficiently define the impacts of dam failure for selecting an

appropriate IDF will vary with the extent of existing and potential downstream development, the

size of the reservoir (depth and storage volume), and type of dam. Evaluation of the river reach

and areas impacted by a dam failure should proceed only until sufficient information is generatedto reach a sound decision, or until there is a good understanding of the consequences of failure.

In some cases, it may be apparent from a field inspection or a review of aerial photographs,Flood Insurance Rate Maps, and recent topographic maps, that consequences attrib-utable to damfailure would occur and be unacceptable. In other cases, detailed studies, including dambreak

analyses, will be required. It may also be necessary to perform field surveys to determine the

basement and first floor elevations of potentially affected habitable structures (such as residentialand commercial).

When conducting dambreak studies, the consequences of the incremental increase due to failureunder both normal (full reservoir with normal streamflow conditions prevailing) and floodflow

conditions up to the point where a dam failure would no longer significantly increase the threat

to life or property should be considered. For each flood condition, water surface elevations with

and without dam failure, flood wave travel times, and rates of rise should be determined. Thisevaluation is known as an incremental hazard evaluation (See Appendix B, Flowchart 2). Sincedambreak analyses and flood routing studies do not provide precise results, evaluation of the

consequences of failure should be conservative.

The type of dam and the mechanism that could cause failure require careful consideration if a

realistic breach is to be assumed. Special consideration should be given to the following factors:

• Size and shape of the breach

• Time of breach formation

• Hydraulic head

• Storage in the reservoir

• Reservoir inflow

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In addition, special cases where a dam failure could cause domino-like failure of downstreamdams resulting in a cumulative flood wave large enough to cause a threat should be considered.

The area affected by dam failure during a given flow condition on a river is the additional areainundated by the incremental increase in flood elevation due to failure over that which would

occur normally by flooding without dam failure. The area affected by a flood wave resulting

from a theoretical dam breach is a function of the height of the flood wave and the downstreamdistance and width of the river at a particular location. An associated and important factor is the

flood wave travel time. These elements are primarily a function of the rate and extent of dam

failure, but also are functions of channel and floodplain geometry and roughness and channelslope.

The flood wave should be routed downstream to the point where the incremental effect of a

failure will no longer result in unacceptable additional consequences. When routing a

dambreak flood through the downstream reaches, appropriate concurrent inflows should be

considered in the computations. Downstream concurrent inflows can be determined using one ofthe following approaches:

• Concurrent inflows can be based on historical records if these records indicate that the

tributaries contributing to the flood volume are characteristically in a flood stage at the

same time that flood inflows to the reservoir occur. Concurrent inflows based onhistorical records should be adjusted so they are compatible with the magnitude of the

flood inflow computed for the dam under study.

• Concurrent inflows can be developed from flood studies for downstream reaches when

they are available. However, if these concurrent floods represent inflows to a downstream

reservoir, suitable adjustments must be made to properly distribute flows among thetributaries.

• Concurrent inflows may be assumed equal to the mean annual flood (approximately

bankfull capacity) for the channel and tributaries downstream from the dam. The mean

annual flood can be determined from flood flow frequency studies. As the distancedownstream from the dam increases, engineering judgment may be required to adjust the

concurrent inflows selected.

In general, the study should be terminated when the potential for loss of life and property

damage caused by routing floodflows appears limited. This point could occur when the following

takes place:

• There are no habitable structures, and anticipated future development in the floodplain islimited.

• Floodflows are contained within a large downstream reservoir.

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• Floodflows are confined within the downstream channel.

• Floodflows enter a bay or ocean.

The failure of a dam during a particular flood may increase the area flooded and also alter the

flow velocity and depth of flow as well as the rate of rise of flood flows. These changes in floodflows could also affect the amount of damage. To fully evaluate the hazard created by a dam, a

range of flood magnitudes needs to be examined. Water surface profiles, flood wave travel times,

and rates of rise should be determined for each condition.

The results of the downstream routing should be clearly shown on inundation maps with the breach-wave travel time, maximum depth of flow, and maximum velocity, indicated at critical

downstream locations. The inundation maps should be developed at a scale sufficient to identify

downstream habitable structures within the impacted area. The current recommended method

for dambreak analysis is the unsteady flow and dynamic routing method used in the

National Weather Service (NWS) model.

Most of the methods used for estimating dambreak hydrographs, including the widely used NWS

model, required selecting the size, shape, and time of formation of the dam breach as input parameters for the computations. Therefore, sensitivity analyses are considered necessary.

Sensitivity analyses, based on varying flood inflow conditions and breach parameters, should be

performed only to the extent necessary to make a decision.

Dambreak studies should be performed in accordance with accepted Guidelines. Refer to

Appendix A for a list of sources of information related to dambreak studies.

6. Incremental Hazard Evaluation for Inflow Design Flood Determination. The appropriateIDF for a project is selected based on the results of the incremental hazard evaluation. This

evaluation involves simulating a dam failure during normal and floodflow conditions and routing

the water downstream. The additional down-stream threat due to the incremental increase inwater surface elevation from dam failure is assessed for each failure scenario.

To evaluate the incremental increase in consequences due to dam failure, begin with the normalinflow condition and the reservoir at normal full reservoir level with normal streamflow

conditions prevailing. That condition should be routed through the dam and downstream areas,

with the assumption that the dam remains in place. The same flow should then be routed throughthe dam with the assumption that the dam fails.

The incremental increase in downstream water surface elevation between the with-failure and

without-failure conditions should then be determined, i.e., how much higher would the water

downstream be if the dam failed than if the dam did not fail? The consequences that could result

should then be identified. If the incremental rise in flood water downstream results inunacceptable additional consequences, assess the need for remedial action.

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If the study under normal flow conditions indicates no adverse consequences, the same analysesshould be done for several larger flood levels to determine the greatest unacceptable

consequences. Under each incrementally larger inflow condition, identify the consequences of

failure. For each larger assumed flood inflow condition (which can be percentages of the

probable maximum flood (PMF)):

• assume the dam remains in place during the nonfailure conditions; and

• assume the dam fails when the peak reservoir elevation is attained for the assumed inflow

condition.

It is not appropriate to assume that a dam fails on the rising limb of the inflow hydrograph.

For example, current methods cannot accurately determine the extent of overtopping that anearth dam can withstand or how rapidly the dam will erode and ultimately breach from

overtopping. Therefore, until such methodologies are available and proven, a conservative

approach should be followed which assumes that failure occurs at the peak of the floodhydrograph. The assumption should also be made that the dam has been theoretically modified tocontain or safely pass all lower inflow floods. This is an appropriate assumption because this

procedure requires that the dambreak analyses start at the normal operating condition, with

incremental increases in the flood inflow condition for each subsequent failure scenario up to the point where a failure no longer constitutes unacceptable additional consequences. In summary,

before selecting larger floods for analysis, determine what failure at a lower flood constituted a

threat to downstream life and property.

The above procedure should be repeated until the flood inflow condition is identified such that a

failure at that flow or larger flows (up to the PMF) will no longer result in unacceptable

additional consequences. The resultant flood flow is the IDF for the project. The maximum IDFis always the PMF, but in many cases the IDF will be substantially less than the PMF.

A PMF should be determined if it is needed for use in the evaluation. If a PMF value is already

available, it should be reviewed to determine if it is still appropriate. The probable maximum precipitation (PMP) for the area should be determined either through the use of the

Hydrometeorological Reports (HMR's) developed by the NWS or through the services of aqualified hydrometeorologist. In addition, the hydrologic characteristics of the drainage basin

that would affect the runoff from the PMP into the reservoir should be determined. After this

information is evaluated, the PMF can be determined.

Once the appropriate IDF for the dam has been selected (whether it is the PMF or somethingless), it should then be determined whether the dam can safely withstand or pass all floodflowsup through the IDF. If it can, then no further evaluation or action is required. If it cannot, then

measures must be taken to enable the project to safely accommodate all floods up through the

IDF to alleviate the incremental increase in unacceptable additional consequences a failure mayhave on areas downstream.

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It is important to investigate the full range of flood flow conditions to verify that a failure underflood flows larger than the selected IDF up through the PMF will not result in any additional

hazard. In addition, once the design for remedial repairs is selected, the IDF should be verified

for that design.

Appendix B provides specific guidance and procedures, including a comprehensive

flowchart, for conducting an incremental hazard evaluation to select the appropriate IDF

for a dam and determine the need for remedial measures.

7. Criteria for Selecting the Inflow Design Flood. Ideally, dams should be able to safely

accommodate floodflows in a manner that will not increase the danger to life and propertydownstream. However, this situation is not always the case, and may not always be achievable.

There are various methods or reasons for selecting the inflow design flood and determining

whether the dam can safely accommodate the flood. The method chosen may be determined by

the amount of time and/or funds available to conduct an evaluation. For example, if time andfunds are scarce, a conservative inflow design flood (e.g., the PMF) can be selected.

Sometimes, inflow design flood selection is straightforward, i.e., given certain criteria, a specific

inflow design flood must be used, due to political decisions and policies established by

government agencies. For example, statutes may require that a flood such as the PMF, a specific

fraction of the PMF, or a flood of specific frequency be selected for a dam with a certain hazardclassification. Fortunately, the most widely accepted approach involves incremental hazard

evaluations to identify the appropriate IDF for a dam.

There is not a separate IDF for each section of a dam. A dam is assigned only one IDF, and it is

determined based on the consequences of failure of the section of the dam that creates thegreatest hazard potential downstream. This should not, however, be confused with the design

criteria for different sections of a dam which may be based on the effect of their failure on

downstream areas.

The PMF should be adopted as the IDF in those situations where consequences attributable to

dam failure for flood conditions less than the PMF are unacceptable.

A flood less than the PMF may be adopted as the IDF in those situations where the consequencesof dam failure at flood flows larger than the selected IDF are acceptable. In other words, where

detailed studies conclude that the risk is only to the dam owners' facilities and no increased

damage to downstream areas is created by failure, a risk-based approach is acceptable.Generally, acceptable consequences exist when evaluation of the area affected indicates thefollowing:

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• There are no permanent human habitations, known national security installations, or

commercial or industrial developments, nor are such habitations or commercial orindustrial developments projected to occur within the potential hazard area in the

foreseeable future.

• There are permanent human habitations within the potential hazard area that would beaffected by failure of the dam, but there would be no significant incremental increase

in the threat to life or property resulting from the occurrence of a failure during

floods larger than the proposed IDF. For example, if an impoundment has a small

storage volume and failure would not add appreciably to the volume of the outflow flood

hydrograph, it is likely that downstream inundation would be essentially the same with or

without failure of the dam.

The consequences of dam failure may not be acceptable if the hazard potential to thesehabitations is increased appreciably by the failure flood wave or level of inundation. When a

dambreak analysis shows downstream incremental effects of approximately two feet or

more, engineering judgment and further analysis will be necessary to finally evaluate the

need for modification to the dam. In general, the consequences of failure are considered

acceptable when the incremental effects (depth) of failure on downstream structures are

approximately two feet or less. However, the two-foot increment is not an absolute

decision-making point. Sensitivity analyses and engineering judgment are the tools used in

making final decisions. For example, if it is determined that a mobile home sitting on blocks

can be moved and displaced by as little as six inches of water, then the acceptable incremental

impact would be much less than two feet. As a second example, if a sensitivity analysisdemonstrates that the largest breach width recommended is the only condition that results in an

incremental rise of two feet, then engineering judgment becomes necessary to determine whether

a smaller breach having acceptable consequences of failure is more realistic for the givenconditions, e.g., flow conditions, characteristics of the dam, velocity in vicinity of structures,

location, and type of structures.

In addition, selection of the appropriate magnitude of the IDF may include consideration of

whether a dam provides vital community services such as municipal water supply or energy.Therefore, a higher degree of protection may be required against failure to ensure those services

are continued during and following extreme flood conditions when alternate services are

unavailable. If losing such services is economically acceptable, the IDF can be less conservative.However, loss of water supply for domestic purposes may not be an acceptable public health

risk.

Flood frequency and risk-based analyses may be used to hold operation and maintenance

costs to a reasonable level, to maintain public confidence in owners and agencies

responsible for dam safety, and to be in compliance with local, state, or other regulations

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applicable to the facility. Generally, it would not be an appropriate risk to design a dam havinga potential for failure at a flood frequency of less than once in 100 years.

When determining the effect flood inflows will have on dam safety, a hydrologic approach may be used. Simply stated, when determining the effect flood inflows will have on dam safety, the

following approach establishes the IDF for the project, and either:

• determines whether an existing project can safely accommodate the IDF; or

• determines how a new project will be designed to safely accommodate the IDF.

Because the entire spectrum of floods up to the PMF level generally needs to be considered inselecting the IDF, it is usually necessary to determine the PMP magnitudes and to develop the

PMF based on that information.

The effort involved in conducting PMP and PMF analyses is quite detailed. Depending on thesignificance of the study being pursued, these analyses should be directed by an engineer trained

and experienced in this specialized field.

The incremental hazard evaluation procedure presented in the previous section is the most direct

method for selecting an inflow design flood. However, there are times when selection becomesdifficult and it may be necessary to conduct further analyses with a risk-based approach. The

incremental hazard evaluation is, in essence, a risk-based approach.

C. PMFs for Dam Safety. The PMF is the upper limit of floods to be considered when selecting

the appropriate IDF for a dam.

1. General. A deterministic methodology should be used to determine the PMF. In thedeterministic methodology, a flood hydrograph is generated by modeling the physical

atmospheric and drainage basin hydrologic and hydraulic processes. This approach attempts to

represent the most severe combination of meteorologic and hydrologic conditions consideredreasonably possible for a given drainage basin. The PMF represents an estimate of the upper

limit of run-off that is capable of being produced on the watershed. Refer to Appendix A for a

list of sources of information related to flood studies.

2. PMP. The concept that the PMP represents an upper limit to the level of precipitation theatmosphere can produce has been stated in many hydrometeorological documents. The

commonly used approach in deterministic PMP development for non-orographic regions is to

determine the limiting dew point temperature at the surface (used to obtain the moisture

maximization factor) and to collect a "sufficient" sample of extreme storms. These extremestorms are moved to other parts of the study area. The latter are done through a method known as

storm transposition, i.e., the adjustment of moisture observed in a storm at its actual site of

occurrence to the corresponding moisture level at the site from which the PMP is to bedetermined. Storm transposition is based on the concept that all storms within a meteorologically

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homogeneous region could occur at any other location within that region with appropriateadjustments for effects of elevation and moisture supply. The maximized transposed storm

values are then enveloped both depth-durationally and depth-areally to obtain PMP estimates for

a specific basin. Several durations of PMP should be considered to ensure the most appropriate

duration is selected.

In orographic regions, where local influences affect the delineation of meteorologicalhomogeneity, transposition is generally not permitted. Alternative procedures are offered for

these regions that are less reliant on the adequacy of the storm sample. Most of these procedures

involve development of both nonorographic and orographic components (sometimes anorographic intensification factor is used) of the PMP. Orographic and nonorographic PMPs are

then combined to obtain total PMP estimates for an orographic basin. Currently, generalized

PMP estimates are available for the United States (see Figure 3).

As our understanding and the availability of data increases, the "particular" PMP estimates that

appear in NWS HMRs may require adjustment to better define the conceptual PMP for a specificsite. Therefore, it may be appropriate to refine PMP estimates with site-specific or regional

studies. The results of available research, such as that developed by the Electric Power Research

Institute, should be considered in performing site-specific studies. Because these studies can become very time consuming and costly, the benefit of a site-specific study must be considered

carefully. Currently, generalized NWS PMP estimates are available for the entire conterminous

United States, as well as Alaska, Hawaii, and Puerto Rico.

D. Floods To Protect Against Loss of Benefits During the Life of the Project - Applicable

Only to Low-Hazard Dams

Dams identified as having a low-hazard potential should be designed to at least meet a minimum

standard to protect against the risk of loss of benefits during the life of the project, hold operationand maintenance costs to a reasonable level, maintain public confidence in owners and agenciesresponsible for dam safety, and be in compliance with local, state, or other regulators applicable

to the facility. Floods having a particular frequency may be used for this analysis. In general, itwould not be appropriate to design a dam having a low-hazard potential for a flood having an

average return frequency of less than once in 100 years.

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IV. ACCOMMODATING INFLOW DESIGN FLOODS

A. Flood Routing Guidelines

1. General. Site-specific considerations should be used to establish flood routing criteria foreach dam and reservoir. The criteria for routing any flood should be consistent with the reservoir

regulation procedure that is to be followed in actual operation. The general guidelines to be used

in establishing criteria are presented below and should be used if applicable.

2. Guidelines for Initial Elevations. In general, if there is no allocated or planned flood controlstorage (i.e., run-of-river), the flood routing usually begins with the reservoir at the normal

maximum pool elevation. If regulation studies show that pool levels would be lower than thenormal maximum pool elevation during the critical inflow design flood (IDF) season, then the

results of those specific regulation studies should be analyzed to determine the appropriate initial

pool level for routing the IDF.

3. Reservoir Constraints. Flood routing criteria should recognize constraints that may exist on

the maximum desirable water surface elevation. A limit or maximum water surface reachedduring a routing of the IDF can be achieved by providing spillways and outlet works with

adequate discharge capacity. Backwater effects of floodflow into the reservoir must specifically

be considered when constraints on water surface elevation are evaluated. Reservoir constraintsmay include the following:

• Topographic limitations on the reservoir stage which exceed the economic limits of dike

construction; public works around the reservoir rim, which are not to be relocated, such

as water supply facilities and sewage treatment plants; dwellings, factories, and other

developments around the reservoir rim, which are not to be relocated. If there is a loss ofstorage capacity caused by sediment accumulation in portions of the reservoir, then this

factor should be accounted for in routing the IDF. Sediment deposits in reservoir

headwater areas may build up a delta, which can increase flooding in that area, as well asreduce flood storage capacity, thereby having an effect on routings.

• Geologic features that may become unstable when inundated and result in landslides,

which would threaten the safety of the dam, domestic and/or other developments, ordisplace needed storage capacity.

• Flood plain management plans and objectives established under federal or state

regulations and/or authorities.

4. Reservoir Regulation Requirements. Considerations to be evaluated when establishing floodrouting criteria for a project include the following:

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• Regulation requirements to meet project purposes, the need to impose a maximum

regulated release rate to prevent flooding or erosion of downstream areas and control rateof drawdown, the need to provide a minimum regulated release capacity to recover flood

control storage for use in regulating subsequent floods, the practicability of evacuatingthe reservoir for emergencies and for performing inspection, maintenance, and repair.Spillways, outlet works, penstocks, and navigation locks for powerplants are sized to

satisfy project requirements and must be operated in accordance with specific instructions

if these project works are relied upon to make flood releases, subject to the following

limitations:

o Only those release facilities which can be expected to operate reliably under theassumed flood condition should be assumed to be operational for flood routing.

Reliability depends upon structural competence and availability for use.Availability and reliability of generating units for flood release during major

floods should be justified. Availability of a source of auxiliary power, for gate

operation, effects of reservoir debris on operability and discharge capacity ofgates and other facilities, accessibility of controls, design limits on operating

head, reliability of access roads, and availability of operating personnel at the site

during floods are other factors to be considered in determining whether to assume

release facilities are operational. A positive way of making releases to the naturalwatercourse by use of a bypass or wasteway must be available if canal outlets are

to be considered available for making flood releases. Bypass outlets for

generating units may be used if they are or can be isolated from the turbines bygates or valves.

o In flood routing, assumed releases are generally limited to maximum values

determined from project uses, by availability of outlet works, tailwater conditionsincluding effects of downstream tributary inflows and wind tides, and

downstream nondamaging discharge capacities until allocated storage elevationsare exceeded. When a reservoir's capacity in regulating flows is exceeded, then

other factors, particularly dam safety, will govern releases. During normal flood

routing, the rate of outflow from the reservoir should not exceed the rate of inflowuntil the outflow begins to exceed the maximum project flood discharge capacity

at normal pool elevation, nor should the maximum rate of increase of outflow

exceed the maximum rate of increase of inflow. This is to prevent outflowconditions from being more severe than pre-dam conditions. An exception to the

above would be where streamflow forecasts are available and pre-flood releases

could reduce reservoir levels to provide storage for flood flows.

5. Evaluation of Domino-like Failure. If one or more dams are located downstream of the site

under review, the failure wave should be routed downstream to determine if any of the

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downstream dams would breach in a domino-like action. The flood routing of flows entering themost upstream of a series of such dams may be either dynamic or level pool. The routing through

all subsequent downstream reservoirs should be a dynamic routing. Tailwater elevations should

consider the effect of backwater from downstream constrictions.

B. Spillway and Flood Outlet Selection and Design

1. General. Spillways and flood outlets should be designed to safely convey major floods to thewatercourse downstream from the dam. They are selected for a specific dam and reservoir on the

basis of release requirements, topography, geology, dam safety, and project economics.

2. Gated or Ungated Spillways. An ungated spillway releases water when-ever the reservoir

elevation exceeds the spillway crest level. A gated spillway can regulate releases over a broad

range of water levels.

Ungated spillways are more reliable than gated spillways. Gated spillways provide greater

operational flexibility. Operation of gated spillways and/or their regulating procedures shouldgenerally ensure that the peak flood outflow does not exceed the natural downstream flow that

would occur without the dam.

The selection of a gated or ungated type of spillway for a specific dam depends upon site

conditions, project purposes, economic factors, costs of operation and maintenance, the IDFitself, and other considerations.

The following paragraphs focus on considerations that influence the choice between gated and

ungated spillways:

•

Discharge capacity - For a given spillway crest length and maximum allowable watersurface elevation, a gated spillway can be designed to release higher discharges than anungated spillway because the crest elevation may be lower than the normal reservoir

storage level. This is a consideration when there are limitations on spillway crest length

or maximum water surface elevation.

• Project objectives and flexibility - Gated spillways permit a wide range of releases and

have capability for pre-flood drawdown.

• Operation and maintenance - Gated spillways may experience more operational problemsand are more expensive to construct and maintain than ungated spillways. Constant

attendance or several inspections per day, by an operator during high water levels, ishighly desirable for reservoirs with gated spillways, even when automatic or remotecontrols are provided. During periods of major flood inflows, the spillway should be

closely monitored. Gated spillways are more subject to clogging from debris and

jamming from ice, whereas properly designed ungated spillways are basically free fromthese problems. Gated spillways require regular maintenance and periodic testing of gate

operations. However, ungated spillways can have flashboards, trip gates, and stop log

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sections, which can have operational problems during floods and may require constantattendance or several inspections per day during high water levels.

• Reliability - The nature of ungated spillways reduces dam failure potential associatedwith improper operation and maintenance. Where forecasting capability is unreliable, or

where time from the beginning of runoff to peak inflow is only a few hours, ungated

spillways are more reliable, particularly for high-hazard structures. Consequences of

failure of operation equipment or errors in operation can be severe for gated spillways.

• Data and control requirements - Operational decisions for gated spillways should have

real time hydrologic and meteorologic data to make proper regulation possible.

• Emergency evacuation - Unless ungated spillways have removable sections, such as

flashboards, trip gates, or stop log sections, they cannot be used to evacuate a reservoir

during emergencies. The capability of gated spillways to draw down pools from the top

of the gates to the spillway crest can be an advantage when emergency evacuation toreduce head on the dam is a concern.

• Economics and selection - Designs to be evaluated should be technically adequatealternatives. Economic considerations often indicate whether gated or ungated spillways

are selected. The possibility of selecting a combination of more than one type of spillway

is also a consideration. Final selection of the type of crest control should be based on acomprehensive analysis of all pertinent factors, including advantages, disadvantages,

limitations, and feasibility of options.

3. Design Considerations. Dams and their appurtenant structures should be designed to give

satisfactory performance. These Guidelines identify three specific classifications of spillways(service, auxiliary, and emergency) and outlet works that are used to pass floodwaters, each

serving a particular function. The following paragraphs discuss functional requirements.

Service spillways should be designed for frequent use and should safely convey releases from a

reservoir to the natural watercourse downstream from the dam. -Considerations must be given to

waterway free-board, length of stilling basins, if needed, and amount of turbulence and other performance characteristics. It is acceptable for the crest structure, discharge channel (e.g., chute,

conduit, tunnel), and energy dissipator to exhibit marginally safe performance characteristics for

the IDF. However, they should exhibit excellent performance characteristics for frequent andsustained flows, such as up to the 1-percent chance flood event. Other physical limitations may

also exist that have an effect on spillway sizing.

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Auxiliary spillways are usually designed for infrequent use and it is acceptable to sustain limiteddamage during passage of the IDF. The design of auxiliary spillways should be based on

economic considerations and should be subject to the following requirements:

•

The auxiliary spillway should discharge into a watercourse sufficiently separated fromthe abutment to preclude abutment damage and should discharge into the main stream a

sufficient distance downstream from the toe of the dam so that flows will not endanger

the dam's structural integrity or usefulness of the service spillway. The auxiliary spillwaychannel should either be founded in competent rock or an adequate length of protective

surfacing should be provided to prevent the spillway crest control from degrading to the

extent that it results in an unacceptable loss of conservation storage or a largeuncontrolled discharge which exceeds peak inflow.

Emergency spillways may be used to obtain a high degree of hydrologic safety with minimal

additional cost. Because of their infrequent use, it is acceptable for them to sustain significant

damage when used and they may be designed with lower structural standards than those used for

auxiliary spillways.

An emergency spillway may be advisable to accommodate flows resulting from misoperation or

malfunction of other spillways and outlet works. Generally, they are sized to accommodate aflood smaller than the IDF. The crest of an emergency spillway should be set above the normal

maximum water surface (attained when accommodating the IDF) so it will not overflow as a

result of reservoir setup and wave action. The design of an emergency spillway should be subjectto the following limitations:

• The structural integrity of the dam should not be jeopardized by spillway operation.

Large conservation storage volumes should not be lost as a result of degradation of the

crest during operation. The effects of a downstream flood resulting from uncontrolled

release of reservoir storage should not be greater than the flood caused by the IDF

without the dam.

Outlet works used in passing floods and evacuating reservoir storage space should be designedfor frequent use and should be highly reliable. Reliability is dependent on foundation conditions,

which influence settlement and displacement of waterways, on structural competence, onsusceptibility of the intake and conduit to plugging, on hydraulic effects of spill-way discharge,

and on operating reliability.

C. Freeboard Allowances1. General. Freeboard provides a margin of safety against overtopping failure of dams. It is

generally not necessary to prevent splashing or occasional overtopping of a dam by waves underextreme conditions. However, the number and duration of such occurrences should not threaten

the structural integrity of the dam, interfere with project operation, or create hazards to

personnel. Freeboard provided for concrete dams can be less conservative than for embankment

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dams because of their resistance to wave damage or erosion. If studies demonstrate that concretedams can withstand the IDF while overtopped without significant erosion of foundation or

abutment material, then no freeboard should be required for the IDF condition. Special

consideration may be required in cases where a powerplant is located near the toe of the dam.

The U.S. Bureau of Reclamation has developed guidelines (See Appendix A) that provide

criteria for freeboard computations.

Normal freeboard is defined as the difference in elevation between the top of the dam and the

normal maximum pool elevation. Minimum freeboard is defined as the difference in pool

elevation between the top of the dam and the maximum reservoir water surface that would resultfrom routing the IDF through the reservoir. Intermediate freeboard is defined as the difference

between intermediate storage level and the top of the dam. Intermediate freeboard may be

applicable when there is exclusive flood control storage.

2. Freeboard Guidelines. Following are guidelines for determining appropriate freeboard

allowances:

• Freeboard allowances should be based on site-specific conditions and the type of dam

(concrete or embankment). Both normal and minimum freeboard requirements should be

evaluated in determining the elevation of the top of the dam. The resulting higher top of

dam elevation should be adopted for design. Freeboard allowances for wind-wave actionshould be based upon the most reliable wind data available that are applicable to the site.

The significant wave should be the minimum used in determining wave runup, and the

sum of wind setup and wave runup should be used for determining requirements for thiscomponent of freeboard.

•

Computations of wind-generated wave height, setup, and runup should incorporateselection of a reasonable combined occurrence of pool level, wind velocity, winddirection, and wind durations based on site-specific studies. It is highly unlikely that

maximum winds will occur when the reservoir water surface is at its maximum elevation

resulting from routing the IDF, unless the reservoir capacity is small compared to floodvolume because the maximum level generally persists only for a relatively short period of

time (a few hours). Consequently, winds selected for computing wave heights should be

appropriate for the short period the pool would reside at or near maximum levels. Normal pool levels persist for long periods of time. Consequently, maximum winds should be

used to compute wave heights.

•

Freeboard allowance for settlement should be applied to account for consolidation offoundation and embankment materials when uncertainties exist in computational methods

or data used yield unreliable values for camber design. Freeboard allowance forsettlement should not be applied where an accurate determination of settlement can be

made and is included in the camber. Freeboard allowance for embankment dams for

estimated earthquake-generated movement, resulting seiches, and permanent

embankment displacements or deformations should be considered if a dam is located in

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an area with potential for intense seismic activity. Reduction of freeboard allowances onembankment dams may be appropriate for small fetches, obstructions that impede wave

generation, special slope and crest protection, and other factors.

•

Freeboard allowance for wave and volume displacement due to potential landslides,which cannot be economically removed or stabilized, should be considered if a reservoir

is located in a topographic setting where the wave or higher water resulting from

displacement may be destructive to the dam or may cause serious downstream damage.

• Total freeboard allowances should include only those components of freeboard which can

reasonably occur simultaneously for a particular water surface elevation. Components of

freeboard and combinations of those components, which have a reasonable probability ofsimultaneous occurrence, are listed in the following paragraphs for estimating minimum,

normal, and intermediate freeboards. The top of the dam should be established toaccommodate the most critical combination of water surface and freeboard components

from the following combinations.

For minimum freeboard combinations, the following components, when they can reasonably

occur simultaneously, should be added to determine the total minimum freeboard requirement:

• Wind-generated wave runup and setup for a wind appropriate for the maximum reservoir

stage for the IDF.

• Effects of possible malfunction of the spillway and/or outlet wor

Federal Guidelines for Dam Safety.pdf

Documents

Transcript of Federal Guidelines for Dam Safety.pdf